1. Field of the Invention
The present invention relates generally to the field of cardiovascular assessment and performance. More specifically, the present invention relates to the field of non-invasive assessment of cardiac function based on pressure-volume relation analysis.
2. Description of the Related Art
The pressure-volume framework for assessing cardiac function, specifically the use of an end-systolic pressure-volume relationship (ESPVR) to assess cardiac contractile function is well-known. This analytic method is widely regarded as among the best means for defining cardiac chamber performance, and its interaction with the vascular (venous and arterial) systems. It can be used to both predict the behavior of the cardiovascular system during physiologic changes (such as sudden increases or decreases in blood pressure or circulating blood volume, and the influence of pharmacologic agents typically used to treat patients with heart disease. The major impediment to practical implementation of this analysis for clinical medicine is the difficulty in making all of the necessary measurements, and the requirement for invasive data. Recent advances in pressure-monitoring and Doppler imaging technology have provided methods to estimate arterial vascular properties non-invasively, but ESPVR characterization has remained much more difficult. A key contributor to this difficulty is the requirement of collecting data from many cardiac cycles for which the "loading" conditions (filling volumes, ejecting pressures) of the heart have been altered.
There have been attempts to estimate the ESPVR from single-cardiac cycles, however, these too do not provide a reliable and easily implemented method. The development of single-beat ESPVR estimation methods has been driven by the desire to simplify the loading procedures required to directly measure the ESPVR, and to avoid a need for continuous LV (left ventricular) volume data. In humans, the ESPVR is for all practical purposes a linear relation, which can be defined by a slope (termed either E.sub.max, or the end-systolic elastance, E.sub.es), and a volume axis intercept (V.sub.0). The slope is a measure of chamber stiffness, and can be used to index contractility changes, while the volume axis intercept is a measure of chamber geometry, and can also be used to index contractile performance changes. Two basic approaches have been employed to date. The simplest and most frequently used has been to ignore V.sub.0, and report the ratio of Pes/Ves (end-systolic pressure/volume ratio). This has shown to be inadequate since there can be marked variability in V.sub.0, with often large non-zero intercepts in patients with infarction or dilated cardiomyopathy. An alternative method uses abrupt occlusion of the aorta during isovolumic contraction to determine peak isovolumic pressure, LV volume during ejection is estimated by integrating aortic flow and synchronizing the result with LV pressure. This method yields two pressure-volume points, one for the isovolumic contraction and one for the resting ejecting contraction, and by linking the two points by a line, one can estimate the slope E.sub.max. A modification to this method was first suggested by Sunagawa et al, " . . . Instead of actually occluding the Aorta to determine isovolumic pressure and volume, an approach that could never be used clinically, these investigators provided a method to estimate this pressure-volume (PV) data by mathematical curve fitting of LV data measured during isovolumic contraction and relaxation phases of a steady-state ejecting beat." The fit predicts the peak pressure that would be generated in the absence of ejection, and by drawing a line between this theoretically derived "isovolumic" pressure-volume point and the measured end-systolic PV point from the same ejecting beat it yields the ESPVR estimate. This method has been tested in normal animals and in humans. While providing adequate results, the method required measurements of high fidelity LV pressure and is based on an arbitrary curve fit. Current non-invasive methods for assessing heart contractile function and the interaction between the heart and the arterial system are not easily implemented in a clinical environment.